The FBI said it will investigate additional emails that have surfaced relating to Hillary Clinton's use of a private email system, in a new twist to the US presidential campaign with 11 days to go before election day.

In a letter to several US congressional committee chairmen, Federal Bureau of Investigation Director James Comey said the agency will determine whether the emails contain classified information, adding that it is unclear how significant the new materials may be.

Mr Comey said he "cannot predict how long it will take us to complete this additional work" or how significant the new material might be..

The new emails were discovered after the FBI seized electronic devices belonging to Mrs Clinton's aide Huma Abedin and her husband Anthony Weiner, the New York Times reported today.           

The announcement shook up the campaign for the 8 November election, where Mrs Clinton is the Democratic Party's candidate and the front-runner in opinion polls.

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The FBI spent about a year investigating Mrs Clinton's use of an unauthorised private email server for her work as US secretary of state between 2009 and 2013 after it emerged that there were classified government secrets in some of her emails.

In July, the FBI characterised Mrs Clinton's handling of classified information via email as "extremely careless" but, after a months-long investigation, it recommended that no criminal charges be brought.

The chairman of Mrs Clinton's presidential campaign said that the campaign was "confident" the FBI would again recommend against pressing charges against the former secretary of state relating to her use of a private email server once it wraps up the latest leg of its investigation.

"The (FBI) Director owes it to the American people to immediately provide the full details of what he is now examining," campaign chairman John Podesta said in a statement. 

"We are confident this will not produce any conclusions different from the one the FBI reached in July," he said.

Mr Comey provided no further details today about the nature of the additional emails now being looked into.

"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," Mr Comey wrote in the letter released today.

He said that the FBI "cannot yet assess whether or not this material may be significant."

Mrs Clinton has previously apologised for her server, which was set up in her home in Chappaqua, New York.

She said the arrangement was a mistake, and that she did not knowingly send or receive classified information.

Republicans criticise Clinton over latest FBI move

Today's news was seized upon by Donald Trump, Clinton's Republican rival for the presidency, who has repeatedly sought to cite her email practices as disqualifying her for office.

"I need to open with a very critical breaking news announcement," Mr Trump said at the start of a rally in Manchester, New Hampshire, before describing Mr Comey's letter.

He was drowned out by a chant from the crowd of "Lock her up".

"We must not let her take her criminal scheme into the Oval Office," Mr Trump continued.

"I have great respect for the fact that the FBI and the Department of Justice are now willing to have the courage to right the horrible mistake that they made."

Paul Ryan, the House of Representative Speaker and the most senior elected Republican in Congress, said Mrs Clinton should not get classified briefings until the investigation was over.

"Yet again, Hillary Clinton has nobody but herself to blame," he said in a statement.

"She was entrusted with some of our nation's most important secrets, and she betrayed that trust by carelessly mishandling highly classified information."

Election experts say about 20% of ballots have already been cast, as more Americans vote by mail or go to the polls early.

"A lot of concern about the emails has already been baked into this electoral cake I think. They know she did it, they know it was inappropriate and, failing some sensational revelation on 6 November, it's hard to see that it's going to make that big a difference," said Linda Fowler, a professor of government at Dartmouth College.